I myself lack the experience to present a coherent argument against it, but ask any trans community and you'll get dozens of replies.

There are some objections that I can present on the fly, though. For one, there is no evidence that sex hormone level or this unobservable "brain gender" is the most important factor contributing to digit ratio. The very Wikipedia article you linked claims, among other things, that there is more ethnic variation than gender variation.

And while this is not an argument against the validity of the research, we must be wary of drawing backward inference between observable physical phenomena and correlated social phenomena. As I mentioned, I know transpeople who were depressed when their digit ratio "didn't match the expectation", irrational as it may be. It should be considered, at best, weak evidence.